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The technology behind China's J-20 stealth fighter may have come from a US plane shot down during the Kosovo war, Balkan military sources say.

Adm Davor Domazet-Loso, Croatia's military chief of staff at the time, claims Chinese agents took parts of a downed F-117 stealth jet in 1999.

The F-117 had been shot down by a Serbian anti-aircraft missile during a Nato bombing raid.

China's J-20 stealth fighter had its first test flight earlier this month.

Currently, the United States is the only nation with a fully operational stealth plane.

'Reverse-engineer'

Nighthawks were the world's first stealth fighters - planes that are almost invisible to radar.

When the F-117 was shot down in 1999 during Nato bombing, it was the first time one of the jets had been hit.

Military officials and experts say they believe that some of the parts found themselves in Chinese hands, which allowed China to replicate them to develop similar technologies.

The Pentagon has downplayed concerns over China's stealth fighter "At the time, our intelligence reports told of Chinese agents criss-crossing the region where the F-117 disintegrated, buying up parts of the plane from local farmers," Adm Domazet-Loso told the Associated Press news agency.

"We believe the Chinese used those materials to gain an insight into secret stealth technologies... and to reverse-engineer them," he said.

A senior Serbian military official confirmed that some of the pieces were removed by souvenir collectors, and that some ended up "in the hands of foreign military attaches".

Alexander Neill, head of the Asia Security Programme at the Royal United Services Institute, said the proposition was highly possible, as Serbia and China had a very close relationship during this time and routinely shared intelligence.

"At that time the Chinese had a close relationship with Serbian military intelligence and in that regard - just as an intelligence-sharing relationship - a windfall such as a F-117 would be gold dust, and any modernising military worth its salt would examine anything of that nature extremely closely," he told the BBC.

He said it would be very difficult to know for sure, but that China had during that time a "rapacious appetite" for technology which would help them with their modernisation programme.

"Given the history of Chinese espionage, it is clear that China has had a very covert history of acquisition," he said.

'No interest'

Parts of the F-117 wreckage, including its left wing, cockpit canopy, ejection seat, pilot's helmet and radio, are on show at Belgrade's aviation museum.

Zoran Milicevic, deputy director of the museum, said: "I don't know what happened to the rest of the plane. A lot of delegations visited us in the past, including the Chinese, Russians and Americans. But no-one showed any interest in taking any part of the jet."

China's Chengdu J-20 stealth jet is expected to be operational some time between 2017 and 2019.

Some analysts have claimed this sign of military strength will worry the US government, but the Pentagon has played down concerns over the fighter.

"Developing a stealth capability with a prototype and then integrating that into a combat environment is going to take some time," US director of naval intelligence Vice-Admiral David Dorsett said earlier this month.

China's official military budget quadrupled between 1999 and 2009 as the country's economy grew. In 2010 it stood at $78bn (£50bn).

But the US has by far the largest defence budget in the world at just over $700bn.